Open up a terminal of your choice and type:
sudo apt-get install chromium ( or use synaptic ) then..
sudo apt-get install galternatives then.....
sudo update-alternatives --config x-www-browser
you will now see choices and you should now see chromium ( was option 0 for me.. choose and hit enter ) example:
/usr/bin/chromium-browser 40 auto mode 0
/usr/bin/chromium-browser 40 manual mode 1
/usr/bin/firefox 40 manual mode 2
Now close terminal
Now go into "preferred applications" and viola you'll see chromium, change that to Chromium
Now selecting links in IRC, ect., it will open up Iron Browser and not the default Firefox, and wont open in Chromium as long as you have Iron already opened up!
Oh happy days are here again!! \o/
******( if you do not install chromium b4 doing any of this, you will see no other options in x-www-browser, config editor or preferred applications )******
Hope this helps all the complaints I see all over the place on Iron not setting as default in Mint!
